She banged him.

Quote Originally Posted by FuryFox
If it's not even her prom, I don't understand the attraction for her to go. My girlfriend would certainly never do something with a romantic edge alone with some other male, and that's not even with my voicing my opinion. It's just understood. Same with myself.

So, if she can't explain why she would go to a romantic funtion with someone else rather than spend an evening with her boyfriend who she loves, then that's a pretty big signal right there.
Amidst all of the, "If you love her you trust her!" bullshit, this is the best advice.